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Meteors about the size of the one that streaked through the sky at 42,000 mph and burst over Chelyabinsk in February — and ones even larger and more dangerous — are probably four, five or even seven times more likely to hit the planet than scientists believed before the fireball, according to three studies published Wednesday in the journals Nature and Science.

That means about 20 million space rocks the size of the Chelyabinsk one may be zipping around the solar system, instead of 3 million, NASA scientist Paul Chodas said at a news conference.

Until Chelyabinsk, NASA had looked only for space rocks about 100 feet wide and bigger.

This meteor was only 62 feet across but burst with the force of 40 Hiroshima-type atom bombs, scientists say. Its shock wave shattered thousands of windows, and its flash temporarily blinded 70 people and caused dozens of skin-peeling sunburns just after dawn in icy Russia. More than 1,600 people in all were injured.

Up until then, scientists had figured a meteor causing an airburst like that was a once-in-150-years event, based on how many space rocks have been identified in orbit. But one of the studies now says it is likely to happen once every 30 years or so.

By readjusting how often these rocks strike and how damaging even small ones can be, "those two things together can increase the risk by an order of magnitude," said Mark Boslough, a Sandia National Lab physicist and co-author of one study.

Lindley Johnson, manager of NASA's Near Earth Object program, which scans the heavens for dangerous objects, said the space agency is reassessing what size rocks to look for and how often they are likely to hit. It's also looking into how the government should react if an asteroid is headed for Earth.

In addition, NASA this fall reactivated a dormant orbiting telescope called WISE specifically to hunt for asteroids, Johnson said. And the agency is expanding ground-based sky searches that might give a few extra days' notice of smaller meteors like the Russian one.

At the same time, NASA and the Federal Emergency Management Agency are looking into the need for evacuations in the case of an immediate threat and how to keep the public informed without scaring people.